4.4 Options for fine tuning Details in the Output Code

Most of these options are also available as declarations in the input file
(see Gperf Declarations).

‘-K slot-name’

‘--slot-name=slot-name’

This option is only useful when option ‘-t’ (or, equivalently, the
‘%struct-type’ declaration) has been given.
By default, the program assumes the structure component identifier for
the keyword is ‘name’. This option allows an arbitrary choice of
identifier for this component, although it still must occur as the first
field in your supplied struct.

‘-F initializers’

‘--initializer-suffix=initializers’

This option is only useful when option ‘-t’ (or, equivalently, the
‘%struct-type’ declaration) has been given.
It permits to specify initializers for the structure members following
slot-name in empty hash table entries. The list of initializers
should start with a comma. By default, the emitted code will
zero-initialize structure members following slot-name.

‘-H hash-function-name’

‘--hash-function-name=hash-function-name’

Allows you to specify the name for the generated hash function. Default
name is ‘hash’. This option permits the use of two hash tables in
the same file.

‘-N lookup-function-name’

‘--lookup-function-name=lookup-function-name’

Allows you to specify the name for the generated lookup function.
Default name is ‘in_word_set’. This option permits multiple
generated hash functions to be used in the same application.

‘-Z class-name’

‘--class-name=class-name’

This option is only useful when option ‘-L C++’ (or, equivalently,
the ‘%language=C++’ declaration) has been given. It
allows you to specify the name of generated C++ class. Default name is
Perfect_Hash.

‘-7’

‘--seven-bit’

This option specifies that all strings that will be passed as arguments
to the generated hash function and the generated lookup function will
solely consist of 7-bit ASCII characters (bytes in the range 0..127).
(Note that the ANSI C functions isalnum and isgraph do
not guarantee that a byte is in this range. Only an explicit
test like ‘c >= 'A' && c <= 'Z'’ guarantees this.) This was the
default in versions of gperf earlier than 2.7; now the default is
to support 8-bit and multibyte characters.

‘-l’

‘--compare-lengths’

Compare keyword lengths before trying a string comparison. This option
is mandatory for binary comparisons (see Binary Strings). It also might
cut down on the number of string comparisons made during the lookup, since
keywords with different lengths are never compared via strcmp.
However, using ‘-l’ might greatly increase the size of the
generated C code if the lookup table range is large (which implies that
the switch option ‘-S’ or ‘%switch’ is not enabled), since the length
table contains as many elements as there are entries in the lookup table.

‘-c’

‘--compare-strncmp’

Generates C code that uses the strncmp function to perform
string comparisons. The default action is to use strcmp.

‘-C’

‘--readonly-tables’

Makes the contents of all generated lookup tables constant, i.e.,
“readonly”. Many compilers can generate more efficient code for this
by putting the tables in readonly memory.

‘-E’

‘--enum’

Define constant values using an enum local to the lookup function rather
than with #defines. This also means that different lookup functions can
reside in the same file. Thanks to James Clark <jjc@ai.mit.edu>.

‘-I’

‘--includes’

Include the necessary system include file, <string.h>, at the
beginning of the code. By default, this is not done; the user must
include this header file himself to allow compilation of the code.

‘-G’

‘--global-table’

Generate the static table of keywords as a static global variable,
rather than hiding it inside of the lookup function (which is the
default behavior).

‘-P’

‘--pic’

Optimize the generated table for inclusion in shared libraries. This
reduces the startup time of programs using a shared library containing
the generated code. If the option ‘-t’ (or, equivalently, the
‘%struct-type’ declaration) is also given, the first field of the
user-defined struct must be of type ‘int’, not ‘char *’, because
it will contain offsets into the string pool instead of actual strings.
To convert such an offset to a string, you can use the expression
‘stringpool + o’, where o is the offset. The string pool
name can be changed through the option ‘--string-pool-name’.

‘-Q string-pool-name’

‘--string-pool-name=string-pool-name’

Allows you to specify the name of the generated string pool created by
option ‘-P’. The default name is ‘stringpool’. This option
permits the use of two hash tables in the same file, with ‘-P’ and
even when the option ‘-G’ (or, equivalently, the ‘%global-table’
declaration) is given.

‘--null-strings’

Use NULL strings instead of empty strings for empty keyword table entries.
This reduces the startup time of programs using a shared library containing
the generated code (but not as much as option ‘-P’), at the expense
of one more test-and-branch instruction at run time.

‘--constants-prefix=prefix’

Allows you to specify a prefix for the constants TOTAL_KEYWORDS,
MIN_WORD_LENGTH, MAX_WORD_LENGTH, and so on. This option
permits the use of two hash tables in the same file, even when the option
‘-E’ (or, equivalently, the ‘%enum’ declaration) is not given or
the option ‘-G’ (or, equivalently, the ‘%global-table’ declaration)
is given.

‘-W hash-table-array-name’

‘--word-array-name=hash-table-array-name’

Allows you to specify the name for the generated array containing the
hash table. Default name is ‘wordlist’. This option permits the
use of two hash tables in the same file, even when the option ‘-G’
(or, equivalently, the ‘%global-table’ declaration) is given.

‘--length-table-name=length-table-array-name’

Allows you to specify the name for the generated array containing the
length table. Default name is ‘lengthtable’. This option permits the
use of two length tables in the same file, even when the option ‘-G’
(or, equivalently, the ‘%global-table’ declaration) is given.

‘-S total-switch-statements’

‘--switch=total-switch-statements’

Causes the generated C code to use a switch statement scheme,
rather than an array lookup table. This can lead to a reduction in both
time and space requirements for some input files. The argument to this
option determines how many switch statements are generated. A
value of 1 generates 1 switch containing all the elements, a
value of 2 generates 2 tables with 1/2 the elements in each
switch, etc. This is useful since many C compilers cannot
correctly generate code for large switch statements. This option
was inspired in part by Keith Bostic's original C program.

‘-T’

‘--omit-struct-type’

Prevents the transfer of the type declaration to the output file. Use
this option if the type is already defined elsewhere.

‘-p’

This option is supported for compatibility with previous releases of
gperf. It does not do anything.